Chain grab hooks are conventionally employed to couple a length of chain to another object, or to loop the chain around another object. The chain grab hook has a throat slightly larger than the thickness of a single chain link and a length of about the width of a single chain link. One link of chain is captured in the grab hook throat while being oriented parallel to the throat, while the two neighboring links of the chain on either side of the captured link are transverse to the throat and therefore cannot slide through the hook along the direction of the length of the chain. Such chain grab hooks are particularly useful in heavy equipment applications, where the operator holds the grab hook in one hand while holding either one portion of the chain or an object to be coupled with the other hand, and strikes the hook at a particular link in the chain to instantly fasten the chain to the grab hook. In many cases, one end of the chain is fastened to a clevis provided on an end of the grab hook opposite the throat, enabling the chain to be quickly formed in a loop. Typically, another end of the chain, not fastened to the clevis, is fastened to one heavy object (e.g., a chain hitch of a tractor) and the loop formed in the chain with the grab hook is wrapped around another object to be pulled, for example.
Provided the chain is kept under tension, the captured chain link will not slide out of the chain grab hook throat through the opening in the throat. However, it is not always possible to maintain constant tension on the chain after engagement of the grab hook with one link. Therefore the chain link can slip out of the opening of the grab hook throat during a brief interruption in the chain tension. In order to prevent chain slippage through the throat, a slidable locking pin may be placed across the throat opening. The locking pin may be spring-loaded and retractable, to permit a chain link to be engaged in the grab hook throat. The force of the link passing through the grab hook throat causes the spring-loaded locking pin to temporarily retract until one side of the link has passed the pin, after which the pin returns to the locking position.
One problem with such a locking pin is that the pin is elongate and must be held in an elongate interior hollow passageway of similar diameter and length extending through the body of the grab hook, so that the pin is liable to bind inside the hollow passageway. Such binding may be caused by either a slight deflection of the pin axis from the hollow passageway axis, or grit inside the hollow passageway. For this reason, a mechanism may be provided for manually controlling the position of the locking pin. However, using such a mechanism requires the use of both hands in operating the grab hook.
Another problem is that the locking pin, while being depressed during chain link insertion into the hook, offer significant resistance to insertion and removal of the chain. The link retention force provided by the spring-loaded locking pin must be sufficient to resist unwanted escape of the captured chain link from the grab hook throat. The retention force with which the chain is held in the hook throat after insertion is the same as the force required to insert it, so that insertion must be as difficult as withdrawal, a significant disadvantage. These factors typically make it difficult to use the chain hook unless the user employs both hands to engage the chain in the hook mouth.
What is desired is a chain hook in which the chain can be so easily inserted into the hook mouth that it requires only one hand to quickly and easily insert a chain link into the hook mouth, but yet holds the inserted chain in the hook mouth with a greater force that that required for insertion, and offers no unnecessary friction while inserting the chain, and whose locking device is not liable to binding.